External apical root resorption (EARR) is a common complication of orthodontic treatment. There is data indicating genetic variation can account for at least 50% of the EARR variation in humans, although only two genes have been identified to possibly explain part of the genetic component of this multifactorial trait, eaving other contributing genes to be determined. The applicant proposes to analyze the variation in histological root resorption associated with orthodontic force (RRAOF) among inbred strains of mice and their F1 offspring. The hypothesis is that the RRAOF will not be the same among the inbred mouse strains. Preliminary data supports this working hypothesis, indicating some genetic influence on this response to treatment. Further determination of interstrain differences, as well as effects of RRAOF on F1's of reciprocal crosses of males and females from the most susceptible and most resistant strains, will be pursued to look for modes of inheritance, influence of parental sex, and further delineation of strain RRAOF susceptibility. This will lead to future genetic studies of candidate gene regions based upon selective breeding of hybrid strains depending on the outcome of the present work.